A heater used in a glow plug of an automobile engine or the like is constituted of a resistor having a heat-generating portion, a lead and an insulating base body. The selection and the design of materials for these parts are made such that the resistance of the lead is smaller than the resistance of the resistor.
A joining portion between the resistor and the lead forms a shape change point or a material composition change point. Accordingly, it has been known that, for the purpose of increasing a joining area so as not to be influenced by the difference in thermal expansion due to heat-generation or cooling in use, an interface between the resistor and the lead is formed obliquely when viewed in cross section parallel to the axial direction of the lead (see Patent Literatures 1 and 2, for example).